Radiative Lifetimes and Excitation Mechanisms in Helium

The radiative decay rates of the n¹S(n=4,5), 3¹P, n¹D(n=3,4,5), n³S(n=4,5), n³P(n=3,4,5), and n³D(n=3,4) states of helium have been measured using pulsed, variable-energy electron-impact excitation and a sampling method of detection. The results agree well with previously published measurements. Experimental evidence is presented for the mechanisms involved in the collisional-transfer reactions where atoms in ¹P states excite ³D levels; nF levels (n≥5) are populated by collisional transfer from n¹P levels and in turn cascade to the ³D levels. Possible laser action in the 3¹P→3¹S and 5F→3³D transitions is suggested.